In an electrical submersible pumping unit, an electric submersible motor is typically used to drive an attached centripetal pump. The electrical submersible pump unit is placed down the well bore of an oil well and is used to push oil to the surface.
The motors are typically filled with a highly refined mineral oil or synthetic with high dielectric strength. The design and operating voltage of these motors typically range from 230 volt up to 7,200 volts. Amperage requirement typically varies from 12 to 200 amps. The required horsepower may be achieved by increasing the length, or diameter, of the motor section. Larger single motor assemblies have been known to exceed 100 feet in length and rated up to 400 horsepower, while tandem motors have been known to approach 90 feet in length and have a rating up to 750 horsepower or greater. Other dimensions and power output are also possible.
The motor is made up of rotors, usually about 12 to 18 inches in length, that are mounted on a shaft and separated by a rotor bearing. Rotor bearings keep the rotating components centered in the stator. The high dielectric oil that fills the motor lubricates the bearings and aids in heat transfer. The rotors are located in an electromagnetic field provided by a stator mounted within the housing.
The stator is composed of a multitude of individual laminations that, along with stator windings that include strands of magnet wire, function as electromagnets. The laminations form a hollow cylinder with one pole of each electromagnet facing toward the center. While no physical movement of the stator takes place, electrical movement is created by progressively changing the polarity of the stator poles in such a manner that their combined magnetic field rotates. In an AC motor, this is easily accomplished since reversing the current each half-cycle will automatically change the polarity of each stator pole.
The rotor also has a group of electromagnets arranged in a cylinder with the poles facing the stator poles. The rotor rotates simply by magnetic attraction and repulsion as the rotor poles attempt to follow the rotating electrical field being generated by the stator. Typically, there is no external electrical connection to the rotor. Instead, the current flow through the rotor is induced by a magnetic field created in the stator.
The multitude of stator laminations that are stacked together and inserted inside a motor housing define a plurality of wire orifices or lamination slots that are filled with the stator windings that include strands of magnet wire. The laminations are typically made from steel. Each strand of wire is covered with a high dielectric coating so that the wires will not short out against one another. Also, a high dielectric tubing or slot liner is typically inserted inside the lamination slots to further protect the wire. Electric motor windings tend to vibrate, which can cause the wires to abrade against one another. If the dielectric coating of the wires is rubbed off, then an electrical short will result. To prevent this, the lamination slots have traditionally been filled with varnish or epoxy that encapsulates the wires into a single mass to prevent the wires from rubbing together.
Three phase induction type motors typically generate a lot of heat. The heat must be removed or the motor will quickly overheat and fail. The high dielectric oil that fills the motor aids in heat transfer. The motor is cooled by conducting heat from inside the motor to the outside of the housing where well fluid flows past the motor to carry the heat away via conduction, convection, and radiation. One drawback with using a material to encapsulate the wires is that the encapsulation material impedes heat dissipation.